++i + ++i + ++i in Java vs C

问题

int i=2;
i = ++i + ++i + ++i;

Which is more correct? Java's result of 12 or C = 13. Or if not a matter of correctness, please elaborate.

回答1:

There is nothing like more correct. It is actually undefined and its called Sequence Point Error. http://en.wikipedia.org/wiki/Sequence_point

回答2:

Java guarantees (§15.7.1) that it will be evaluated left-to-right, giving 12. Specifically, ++ has higher precedence that +. So it first binds those, then it associates the addition operations left to right

i = (((++i) + (++i)) + (++i));

§15.7.1 says the left operand is evaluated first, and §15.7.2 says both operands are evaluated before the operation. So it evaluates like:

i = (((++i) + (++i)) + (++i));
i = ((3 + (++i)) + (++i)); // i = 3;
i = ((3 + 4) + (++i)); // i = 4;
i = (7 + (++i)); // i = 4;
i = (7 + 5); // i = 5;
i = 12;

In C, it is undefined behavior to modify a variable twice without a sequence point in between.

回答3:

The Java result makes sense to me because the operators give the result you would expect, but no serious program should contain a statement like this.

EDIT: I'm amused that this one sentence response has been my highest scored answer of the evening (compared to the dozen other answers I posted, some with pages of code samples). Such is life.

回答4:

In C this is undefined behavior. There is no correct behavior.

来源：https://stackoverflow.com/questions/3879176/i-i-i-in-java-vs-c